It has long been an object of silver halide-based color photographic materials to create an image of an object in an accurate manner, both in terms of color and image structure characteristics such as graininess and sharpness. It is well known that the perceived sharpness of photographic images can be degraded through halation effects; that is, the reflection and subsequent diffusion of light within the light capturing element; in particular, reflection from the support. It is well known to use antihalation layers between the support and the sensitized layers in films to reduce light reflection. To be effective, an antihalation layer contains materials that absorb light and prevent reflection. In general, it is highly desirable for the light absorbing materials to be totally removed from the film element (or otherwise made colorless) after development in order to avoid increased background density. One well known type of light absorbing material suitable for use in antihalation layers is colloidal or finely divided elemental or metallic silver (also referred to as `gray` silver). This type of silver metal is in a filamentary form and, is such form, absorbs light across the visible spectrum appearing gray or black. It is generally easily removed from the film element by the normal bleaching and fixing steps used to remove imaging silver from the element. This silver metal is not light sensitive and does not contribute to image formation. For references, see T. H. James, The Theory of the Photographic Process, 4th Edition, p. 579, U.S. Pat. No. 3,434,839, JP 09-067122A2 and Y. J. Zahng et al, Chin. Chem. Lett. 7(7), 687-690(1996).
Another use of colloidal or finely divided elemental or metallic silver is as a blue light absorbing filter. This form, commonly referred to as Carey-Lea silver, differs from `gray` silver by being spherical in form. For references, see F. Evva, J. Signalaufzeichnungmaterialien, 4(1), 43-60(1976) and G. Frens, Kolloid-Z.Z. Polym, 233(1-2), 922-9(1969). This material is generally located in a non-imaging layer (commonly referred to as a yellow filter layer) farther away from the exposing source than or "underneath" the blue light sensitive emulsion layer. The function of this layer is to absorb any blue light not captured by the blue sensitized layers, thus avoiding undesired exposure by blue light of the underlying green and red sensitized emulsion layers, which retain some inherent sensitivity to blue light.
A problem associated with the use of elemental silver in both antihalation and yellow filter layers is an undesired increase in fog in nearby imaging layers. During development, silver ions are released and/or made soluble from the imaging layer. These silver ions can migrate to a non-light sensitive layer where the elemental silver is present. The silver can serve as nuclei for the reduction of the migrating silver ions to silver metal with concurrent oxidation of developer to oxidized developer. This process is called solution physical development (for references, see T. H. James, ibid., Chapter 13) and is non-imagewise. The oxidized developer can diffuse out of the antihalation layer and back into the nearby imaging layer where it can react with the couplers present and form dye in a non-imagewise fashion. This process is often highly process sensitive and can lead to variations in Dmin during photofinishing.
Another problem with the use of elemental silver in non-imaging layers is that these layers can absorb inhibitor fragments and other silver absorbing materials. This results in lower effective concentrations of the free species in the imaging layers. Restricted diffusion of such species through the layer containing the elemental silver can also occur.
It is known that the solution physical development involving elemental silver can be modified by the use of additives. For example, GB 2280276 A1, U.S. Pat No. 3,647,439, DE 1949418, East German Patent 2006 91/6 and Japanese Patent Application (Kokai) JP 3-138639A2 all describe various classes of materials, including thiols of various types, that are useful for controlling the properties of elemental silver. In particular, the '439 and '91/6 references describe among others, the use of various types of heterocyclic thiols for use in non-light sensitive layers. However, in all of these references, the materials are water soluble and, of all the examples of heterocyclic thiols shown, the average ClogP is 1.66 with a maximum ClogP is 3.18 (compound 11 in Table II of '91/6). Such water soluble materials can undesirably diffuse to imaging layers where they can cause inhibition of development and loss of sensitivity to light.
Solution physical development can be promoted by materials that form soluble silver salts. In particular, materials that release low molecular weight water solubilized thiols, which are used as bleach accelerators, can increase the amount of solution physical development. Couplers that release such thiols are known are bleach accelerator releasing couplers; for examples, see EP 193389, U.S. Pat. No. 4,861,701; U.S. Pat. No. 4,959,299; U.S. Pat. No. 4,912,024; U.S. Pat. No. 5,300,406 and U.S. Pat. No. 5,358.828. It is also possible to release the same bleach accelerators from materials other than couplers by imagewise means that do not involve direct coupling with oxidized developer; for example, see U.S. Pat. No. 4,684,604 or by non-imagewise means, for examples, see U.S. Pat. No. 4,923,784, U.S. Pat. No. 4,865,956 and U.S. Pat. No. 5,019,492. Thus, increases in Dmin in imaging layers near to non-imaging layers which contain collodial silver are particularly problematic when bleach accelerators are also present.
Substituted mercaptotetrazoles are commonly known in the art either as inhibitor fragments and as antifoggants. As inhibitor fragments, they are attached to a coupling moiety through a sulfur or nitrogen atom and do not interact with silver until coupling occurs and the sulfur atom is freed; for example, see U.S. Pat. No. 3,227,554 and C. R. Barr et al, Photogr. Sci. Eng., 74, 214 (1969). As part of a DIR, the mercaptotetrazole does not have a free --S--H or --N--H group. Generally, it is desirable that the mercaptotetrazoles released from DIRs are partially water soluble so that they are free to diffuse to other layers to cause interimage. As antifoggants, these materials are generally at least partially water soluble or soluble in water-miscible solvents such as methanol and are added directly to silver emulsions before coating of the film or added directly to the developer solutions. The use of various solubilized mercaptotetrazoles as antifoggants is shown, for example, in Research Disclosure, June 1984, pp 274-278; U.S. Pat. No. 4,952,485; U.S. Pat. No. 5,362,620; U.S. Pat. No. 5,244,779; U.S. Pat. No. 4,963,475; U.S. Pat. No. 3,266,897; Belgian Patent 747,628, UK Patent 1,275,701; JP-05-313282; EP 454149A1; JP-02-207248; EP 509519A2 and EP 337370A2.
EP 0 369 486B1 describes the use of 2-mercapto-benzoxazoles,-benzothiazoles and -benzodiazoles for use in combination with fine silver halide emulsions in a protective layer, where the light sensitivity of the fine silver halide emulsions are more than 1 exposure unit less sensitive to light than the least sensitive silver halide emulsions present in another layer. JP-03-163435A2 discloses the use of mercaptooxadiazole derivatives with virtually non-photosensitive silver halide emulsions. In both of these references, the silver halide emulsions are not elemental silver.
A problem to be solved is to provide a photographic element containing a non-light sensitive layer containing elemental silver which has a reduced tendency to increase the Dmin of nearby light sensitive layers.